Portion of mask



FIG. 1 is a perspective view of a portion of mask according to our design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The use broken lines indicates environmental structure and does not form part of the claimed design. 

The ornamental design for a portion of mask, as shown and described. 